1. Field of the Invention
This invention relates to a control system for an automatic transmission of the type having planetary gearsets and hydraulically actuated brakes and clutches, which selectively hold components of one or more gearsets to produce the various drive ratios of the transmission. More particularly, the present invention relates to a control system having feedback loops for controlling the synchronous operation of hydraulically actuated friction elements and a feedback loop for controlling the output torque of a power source during a power-on gear ratio change.
2. Description of the Prior Art
Hydraulically actuated, automatic transmissions generally include a turbine driven hydrodynamically from the impeller of a torque converter, the turbine transmitting power to a gearset to produce the forward drive and reverse gear ratios. When any forward or reverse gear ratio is selected, though the vehicle is coasting or standing at idle, the engine transmits torque through the torque converter, various clutches and gears to the drive wheels. Automatic transmissions can have the gear ratio changes produced as a result of repetitively executing an algorithm that produces a gear change command signal when the transmission is to be shifted and a signal representing the gear ratio to which the transmission is to be shifted.
The gearing arrangements of automatic transmissions typically include one or more passive overrunning clutches necessary to avoid disc clutch to disc clutch, i.e. synchronous, shifts under power, while the power plant output torque is directly influencing the transmission output, and hence the driver's perception of the shift smoothness. Overrunning clutches are used because of the difficulty in achieving the desired level of active clutch torque control necessary to avoid extremely harsh shifts. Such synchronous shifts are accomplished, according to the method of this invention, by maintaining precise control over the hydraulic pressures that engage and disengage the disc clutches, simplifying the overall gearing arrangement, maintaining precise control over the power source output torque, and using closed loop feedback control on the transmission output variables, thereby eliminating several unnecessary passive clutches.